1. Field of the Invention
The present invention relates to a lithium secondary cell, and more particularly to a lithium secondary cell using lithium for the negative pole whereby a reduction in cell life due to excessive discharge can be improved.
2. Related Background Art
The amount of CO.sub.2 gas contained in the air has been increasing over the past years, and a possibility is presented that the greenhouse effect will cause global warming. Thermal power plants convert thermal energy obtained by burning fossil fuels, etc. into electric power. It is, however, becoming difficult to construct new thermal power plants which could emit large amounts of CO.sub.2 gas or NO.sub.x, hydrocarbons, CO, etc. with burning of fuels. Thus, as an effective utilization method of electric power generated by generators in the thermal power plants, it is proposed that so-called load leveling be used, wherein nighttime power is stored in secondary cells installed in ordinary households and the stored electric power is consumed in the daytime when power consumption is high. For applications to electric vehicles considered not to emit air pollutants including CO.sub.x, NO.sub.x, hydrocarbons, etc., there are demands to develop secondary cells of higher energy density. Further, for applications to light sources for portable equipment such as book-type personal computers, word processors, video cameras, and portable telephones, it is of urgent necessity to develop more compact, lighter, and higher-performance secondary cells.
Such more compact, lighter, and higher-performance secondary cells are under development, for example, like a rocking chair type lithium ion cell using an anode activator of a material obtained by introducing lithium ions into an intercalation compound and a cathode activator of carbon, which is partly being used in practice. Further, development is under way for a lithium secondary cell using a cathode activator of metallic lithium, which can attain higher energy density than the lithium ion secondary cell.
Also used in practice is a lithium secondary cell using a negative-pole material of a metal oxide such as niobium pentoxide (Nb.sub.2 O.sub.5). (The cell of this type has a feature of long cycle life, because metallic lithium is not separated out upon charge. However, it has the disadvantage of lower energy density than that of other lithium secondary cells.)
When these higher-energy-density lithium ion secondary cells and lithium secondary cells are used in the electric vehicles or portable equipment as described above, they are often used as a combinational cell increased in current and voltage by connecting a plurality of cell elements in series or in parallel. In applications as a combinational cell, a cell element with the smallest discharge capacity is always first to be discharged up, because of variations in capacity or variations in cycle life characteristics between the cell elements connected. Therefore, this cell element with the smallest discharge capacity is always excessively discharged before completion of discharge of the other cells. Then this cell element becomes a rate-determining factor and decreases the cycle life. As a result, the life of the combinational cell is also decreased.
Meanwhile, examples of secondary cells already commercially available and used in practice are nickel-cadmium secondary cells and nickel-zinc secondary cells. These are called "aqueous cells", because they use alkali dissolved in water as a solvent of an electrolyte solution. In order to prevent the over discharge or excessive discharge, which is a problem for the lithium ion secondary cell etc., the aqueous cells use a technique of preliminarily adding an activator in a charged state (called a discharge reserve), such as metallic cadmium or metallic zinc, into the cathode activator, thereby suppressing decomposition or the like of the electrolyte solution (electrolysis of water occurs after using up the dischargeable activator in the electrode, thereby generating hydrogen gas from the negative pole) by discharging this activator in the charged state upon excessive discharge.
However, reactions upon charge and discharge, of the cathode activator in the above aqueous cells are repeated between hydroxide and metal (for example, in the case of nickel-cadmium cell, the reactions are as follows: Cd+2OH.sup.- .revreaction.Cd(OH).sub.2 +2e.sup.-), whereas in the case of the lithium secondary cell, lithium ions are transferred through the electrolyte solution between the positive pole (anode) and the negative pole (cathode). Namely, the reaction modes upon charge and discharge are different, and thus, the concept of discharge reserve as in the above aqueous secondary cells cannot be applied to the lithium secondary cell as it is.
When the lithium secondary cell is excessively discharged, excessive lithium ions are inserted into a crystal lattice of the anode activator in the positive pole, which deforms or breaks the lattice. After that, the anode activator results in decreasing its amounts of inclusion and detachment of lithium ions, which could be a cause to decrease the cell life.
In the case of the lithium secondary cell using lithium metal, the negative pole reacts with anions in the electrolyte solution to form a film on the surface of lithium metal, upon excessive discharging which lowers reversibility of inclusion and detachment of lithium. This could be a cause of shortening the cell life. The electrolyte solution is also decomposed to form hydrocarbons and carbonic acid gas, which increases the concentration of the electrolyte solution so as to cause a decrease in conductivity. This could be another cause to decrease the cell life. In the case of the lithium ion secondary cell using carbon for the negative pole, a film formed by a reaction with anions in the electrolyte solution on the carbon surface obstructs insertion and detachment of lithium ions between carbon layers, which could be a cause of shortening the cell life.
Accordingly, there is a need to devise some means against excessive discharge for the lithium secondary cell and the lithium ion secondary cell. It is, however, a present status that, in applications to combinational cells, which are likely to cause excessive discharge in practical use, the countermeasures listed below are employed in order to decrease excessive discharge as much as possible, but neither of them can present a substantial improvement.
(a) To make capacities of cell elements used in a combinational cell even. PA1 (b) To set a high final discharge voltage. PA1 (c) To monitor and control voltages of the respective cell elements.
Therefore, there are demands to develop a lithium secondary cell having a higher energy density, which is free of any decrease in the cell life even if the cell is subjected to excessive discharge.